Logarithms 2

Find the value of a a if 12 log 12 = 3 a log 3 + 2 a log 8 12 \log12 = 3a \log3 + 2a \log 8


The answer is 4.

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4 solutions

12 log 12 = 3 a log 3 + 2 a log 8 12\log12 = 3a\log3 + 2a\log8

12 log 12 = 3 a log 3 + 6 a log 2 \rightarrow 12\log12 = 3a\log3 + 6a\log2

12 log 12 = 3 a log 3 + 3 a log 4 \rightarrow 12\log12 = 3a\log3 + 3a\log4

12 log 12 = 3 a log 12 \rightarrow 12\log12 = 3a\log12

12 = 3 a ( log 12 0 ) \rightarrow 12 = 3a ------------------ (\log12 \not = 0 )

a = 4 \rightarrow a = \boxed{4}

Dang Anh Tu
Apr 6, 2014

12 log 12 = 12 × ( log 3 × log 4 ) 12\log { 12 } =12\times \left( \log { 3 } \times \log { 4 } \right)

12 log 3 + 12 log 4 = 3 a log 3 + 2 a log 8 \rightarrow 12\log { 3 } +12\log { 4 } =3a\log { 3 } + 2a\log { 8 }

( 12 3 a ) log 3 = ( 6 a 24 ) log 2 \rightarrow \left( 12-3a \right) \log { 3 } =\left( 6a-24 \right) \log { 2 }

Because f ( a ) = ( 12 3 a ) log 3 f\left( a \right) =\left( 12-3a \right) \log { 3 } is monotonically decreasing function and g ( a ) = ( 6 a 24 ) log 2 g\left( a \right) =\left( 6a-24 \right) \log { 2 } is monotonically increasing function

So there is only 1 unknown in this equation.

Please draw 2 graphs of f(x) and g(x) and look at these graphs: The answer is a = 4

Hey yo,

as for 12 log 12 = 3a log 3 + 2a log 8

log 12^(12) = log ^(3a) + log 8^(2a)

log 12^(12) = log [ 3^(3a) x 8^(2a)]

12^(12) = 3^(3a) x 8^(2a)

(3 x 4)^12 = 3^(3a) x 8^(2a)

[3^(12)][4^(12)] = 3^(3a) x 8^(2a)

by comparison,

3^(3a) = 3^(12)

3a = 12

a = 12/3 = 4

or

4^(12) = 8^(2a)

2^(24) = 2^(6a)

6a = 24

a = 4

therefore,the answer is a = 4 ,

thanks...

Budi Utomo
Apr 11, 2014

12^12 = 3^3a . 8^2a --> 3^12. 2^24 = 3^3a . 2^6a --> So, a is 4.

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